A 3-mode parameterization of below-cloud scavenging of aerosols for use in atmospheric dispersion models

Atmospheric aerosols are subject to below-cloud scavenging by precipitation. The scavenging coefficient depends on the aerosol size significantly. The traditional bulk parameterization represents the mean wet scavenging coefficient for the whole aerosol size range. This parameterization significantly overestimates the scavenging of aerosol mass by a heavy rain or a long-duration medium rain. In this study, we present a 3-mode parameterization of the mean scavenging coefficient for each aerosol mode instead of representation for the whole aerosol size range. The new parameterization takes into account the aerosol number size distribution, the rain droplet size distribution and the spectral collision efficiency between the aerosol particle and the rain droplet. Comparing the calculation of mass depletion due to below-cloud scavenging, the 3-mode parameterization agrees well with the size-resolved explicit method. The new parameterization can be easily implemented in atmospheric dispersion models.